Constraining the timing of soil formation in linear dunes: A case study from the Strzelecki and Tirari Deserts, central Australia part of Vignettes:Vignette Collection
Desert dune formation can be episodic. The intensity of aeolian activity varies in response to environmental conditions, including concentrated reworking events and periods of prolonged partial activity. Buried soils (paleosols) provide the most unambiguous indicators of past environmental conditions within dune sediments, acting as markers of relative environmental stability, since soil formation (pedogenesis) takes place during breaks in dune building. Paleosols can also act as stratigraphic markers. Arid zone soils are characterised by precipitation of carbonates and gypsum, and clay coatings surrounding sand grains (cutans) (Fitzsimmons et al., 2009). Cutans are formed by the downward transportation of clays by infiltration of rain water. In some areas such as desert margins, soil horizons may be sufficiently distinctive as to isolate events of different age, thus establishing a relative history (chronology) of dune building events (Bowler, 1976). However in relatively more arid or sediment-poor regions, paleosols may be more difficult to distinguish or identify, or may be incompletely preserved due to "reworking" of dune sediments by wind. The potential for incomplete stratigraphic preservation within dunes is a critical consideration for reconstructing past environments. Here we investigate dune stratigraphy and the timing of soil formation in desert dunes from the Strzelecki and Tirari Deserts of central Australia. The palaeosols in central Australian dunes exhibit similar characteristics to one another, meaning that no one period of pedogenesis can be distinguished from another in this region. Several cycles of dune building are preserved within the internal stratigraphy of many of the dunes in the Strzelecki and Tirari Deserts, however not every site contains the same stratigraphy (Figure 1). Successions of multiple sand units and palaeosols indicate that dune reactivation took place over several episodes of heightened aeolian activity. Some sites do not preserve paleosols, and at these sites it is possible that parts of the stratigraphic record were removed by reworking (Munyikwa, 2005). In comparable landscapes which are relatively more sediment-poor, such as the Kalahari Desert of southern Africa, desert dunes do not preserve any evidence of soil formation at all (Stone and Thomas, 2008). Sedimentological analysis can provide information about the degree of reworking of desert dune sediments. This has implications for the preservation of dune stratigraphy. For example, fragmented coatings on grains, indicative of abrasion by aeolian reworking of underlying soil material, are common across the Strzelecki and Tirari Desert dunefields (Fitzsimmons et al., 2009). Reworking may completely remove horizons deposited during earlier arid periods. The removal of horizons from sections of individual dunes also occurs, possibly through limited lateral migration of dune crests. The propensity for sediment reworking also suggests that, in areas where sediment supply is limited, dune records may be biased towards the end of a period of aridity, although there may be dunes which preserve prolonged phases of partial activity and associated slow sediment accretion. Linear dunes have previously been considered inefficient recorders of the history of aridity of a region due to reworking (Munyikwa, 2005), particularly when assessing the palaeoenvironmental record of individual or small numbers of dunes. However, integrated studies from multiple dunes across a region overcome the limitations of sampling bias in individual dune studies, allowing identification of multiple periods of dune activity and soil formation (Fitzsimmons et al., 2007a). The exact timing of soil formation in desert dunes cannot be directly determined by dating techniques. However, the timing of dune activity can be dated using optically stimulated luminescence (OSL) dating techniques. OSL measures when sediments were last exposed to sunlight, and therefore when the sediments were deposited/buried. Periods of dune stabilisation and soil development can therefore be identified where palaeosols occur within time periods devoid of age estimates. In the Strzelecki and Tirari Deserts (Fitzsimmons et al., 2007a), the OSL chronologies of individual dunes which contained multiple palaeosols were used to constrain the timing of pedogenesis (Figure 2). Soil formation must have occurred at some time between the deposition of the stratigraphic unit in which a paleosol formed, and the onset of the overlying unit. Three periods of pedogenesis were identified. Pedogenesis must have occurred some time between 106-73 ka, 60-43 ka and 27-24 ka (Table 1). Regional soil formation, and associated dune stability, was inferred from the overlap of these age ranges (Figure 3). The timing of pedogenesis also corresponds to an absence of ages for dune activity at the other sites. It is also possible to compare the timing of soil formation with that of regional dune activity. Figure 4 shows the estimated timing of pedogenesis relative to the overall dune chronology for the region. It suggests that the periods identified for soil development correspond approximately to breaks in the dune accumulation record across the entire region. Because we cannot directly date paleosols, we cannot constrain the timing of pedogenesis more precisely. This is a feature of any area where reworking plays a significant role in dune formation. Nonetheless, OSL dating can be used as an effective tool to provide at least approximate timing for geomorphic processes such as pedogenesis, in the absence of direct dating techniques.
Regional trends in desert dune morphology and orientation: Examples from the Australian deserts part of Vignettes:Vignette Collection
Dunes are the major landforms within the desert regions of the world, owing largely to the dominance of wind over water as a geomorphic agent in the arid zone. Linear dunes, which form parallel to the resultant vector of sand-shifting winds, are the most abundant desert dune forms globally. In Australia, linear dunes occupy approximately one-third of the continental land surface, and form a large continental-scale whorl (Wasson et al., 1988) (Figure 1A). The shape (morphology) of linear dunes varies significantly within dunefields, a complexity arising from their response to the factors causing their formation. Critical mechanisms include wind regime, sediment availability, physical barriers to migration, and local hydrology. Wind regime influences linear dune orientation. Physical barriers to migration, such as rivers, hills, alluvial fans and active floodplains or groundwater discharge zones, also have the potential to alter the orientation, spacing and level of organisation of a dunefield. Morphologic variability is most visibly manifested in the spacing and density of junctions between dunes, and their orientation. Several models exist for linear dune formation at large scales; for discussion on this topic refer to vignette "Linear dune formation: longitudinal elongation or local wind rifting?". Sediment availability influences dune spacing and size; dune size is sometimes expressed through changes in height, and sometimes by changes in width. For simple linear dunes such as those in the Strzelecki and Tirari Deserts of central Australia (Figure 1), spacing between dunes is a function of sediment supply, dune height is limited and mostly constant, and sediment availability is expressed by spacing and dune width. In simple linear dunefields such as these, spacing can reasonably be used as an indicator of sediment supply, and is related, by extension, to substrate type (Fitzsimmons, 2007; Wasson et al., 1988). The Strzelecki and Tirari Desert dunefields form part of the most spatially variable section of the Australian continental dune whorl (Figure 1A). The orientation of linear dunes reflects the direction of sand-shifting winds prevailing during their formation. Linear dunes typically form within 30° of the resultant wind vector (Tsoar, 1978). The orientation of dunes in the Strzelecki and Tirari Deserts is oblique to the net direction of the current sand-shifting wind regime. In some places, linear dunes lie in secondary orientations oblique to both the dominant dune trend and the present wind regime as overprinting relationships (Figures 1B, 2). Localised areas of intersecting dune trends also occur in the Simpson Desert to the northeast (Hollands et al., 2006) (Figure 1C). The preservation of multiple dune orientations suggests that linear dunes may be reoriented over time in response to significant changes in wind regime. The timing of dunefield reorientation may help us to understand when and how this process occurred, since we can compare what took place in the dunefields with proxies for environmental and climatic change elsewhere in the Australian region. Evidence from dunes in the Simpson Desert indicates that the wind whorl has migrated southward since the Last Glacial Maximum (LGM: ~20 ka) by approximately 1-1.5° (Hollands et al., 2006). In the Simpson Desert, linear dunes trending oblique to the modern wind regime formed between 74-14 ka, while linear dunes which reflect the modern wind regime initiated between 14-10 ka. By comparison, in the Strzelecki Desert, dunes in the dominant orientation formed around 34.7-28.6 ka, suggesting that this orientation was established in this region well before the LGM (Fitzsimmons et al., 2007a). In the northern Tirari Desert, the dominant north-trending linear dunes overprint a more northwest dune orientation (Figure 2A). At TW site, an obliquely oriented dune and a dune in the dominant orientation which clearly overprinted the first were sampled for optically stimulated luminescence (OSL) dating to ascertain the timing of dune formation and reorientation (Fitzsimmons et al., 2007a) (Figure 2B). The lowermost ages for both dunes are older than most linear dunes in the dominant orientation in the region. This age relationship suggests an earlier reorientation of the dunefields in response to changing wind regime than the post-LGM reorganisation of wind patterns in the Simpson Desert. Compared with the lowermost ages of most dunes in the dominant orientation in the region (34.7-28.6 ka) (Fitzsimmons et al., 2007a), dunefield reorientation may have occurred between 65.8-34.7 ka. The proposed mechanisms responsible for dune reorientation vary. Periodic strengthening and expansion of the anticyclonic pressure belt, responsible for the anticlockwise wind system which formed the Australian dune whorl, has been suggested, as has equatorward compression of the anticyclone system. The oblique dune orientations at the TW site do not correspond to a geometry approximating a transposed dune whorl, nor are the dunes here LGM in age, which would be expected in the case of a straightforward latitudinal shift in wind regime. It is possible that the northern Strzelecki and Tirari Desert region corresponded to a zone of directionally variable winds in the centre of the anticyclonic system in the past, as is the case for the present day Simpson Desert area further to the north, implying a more southerly or larger anticyclone in the past. Pre-existing morphology also appears to play a role during periods of reactivation. Dunes in the Strzelecki and Tirari Deserts maintain their original morphology under conditions which would not necessarily be conducive to perpetuating such forms, in a kind of morphologic inertia. Dating of the timing of dune reactivation also points to coeval reactivation of transverse (forming perpendicular to prevailing sand-shifting winds) and linear dunes in the Strzelecki Desert (Fitzsimmons et al., 2007b). It is likely that morphologic inertia also played a role in the coeval reactivation of pre-existing transverse dunes. The relationship between past and present dune orientations is clearly complex, with dune orientations prior to the LGM preserved, and both linear and transverse dunes affected by morphologic inertia following dunefield reorientation.
Linear dune formation: Longitudinal elongation or local wind rifting? An example from the Strzelecki Desert, central Australia part of Vignettes:Vignette Collection
The Australian desert dunefields are dominated by linear dunes, which form a large anti-clockwise whorl across more than one third of the continent (Wasson et al., 1988). Despite being so extensive, the mechanisms of regional scale formation of linear dunes through time and space are poorly understood (see vignette "Regional trends in desert dune morphology and orientation"). Linear dunes form broadly parallel to the resultant vector of sand-shifting winds, and can vary in length from several hundred metres to several hundred kilometres in length. Several models exist for linear dune formation at large scales. The downwind extension model proposes that linear dunes extend longitudinally by progradation along their length (Mabbutt and Sullivan, 1968; Wopfner and Twidale, 2001). In such cases, sediment derived from upwind dominates over locally-derived sediment, resulting in increasingly younger dune ages downwind and sediment evolution. In the windrift model, aeolian scouring ("wind rifting") of sediment from the local interdune swales onto dunes causes upward accretion of sediment, limited by supply (Hollands et al., 2006; King, 1960). The longitudinal morphology of the linear dune is maintained by deflection of grains along dune axes for short distances (Tsoar, 1978), resulting in downwind propagation of the dune morphology. Dune formation by this model would show no decline in age downwind, and would not rely on upwind sources of sediment. The timing of linear dune activity along a longitudinal transect may help us to better understand the process of linear dune formation as a function both of space and time. The region of linear dunes on the south-western margin of the Strzelecki Desert in central Australia, east of the large playa Lake Frome, provides an excellent site to test the proposed models of dune formation, since dunes there preserve multiple stratigraphic horizons separated by paleosols, and in many places overlie fossil transverse source-bordering dunes. The preservation of dune stratigraphy indicates the occurrence of multiple dune building events at different points along longitudinal transects, and the proximity or otherwise of fossil transverse dunes indicates relative availability of sediment locally. The fossil transverse dunes are designated as such, despite not lying immediately adjacent to Lake Frome, since they originally formed as shorelines adjacent to a more easterly ancestral lake which migrated westwards over time. Each fossil transverse dune was preserved following episodes of lake depocentre migration or regression. They are subdued features in the landscape due to degradation over time. Dune orientation does not align exactly with the current wind regime, indicating that the dunefields are largely relict features that formed under wind conditions different from those prevailing today. The role of downwind extension versus local aeolian wind rifting in dune formation was tested by sampling three sites along an approximately 20 km longitudinal transect of an individual dune (Figure 1). The three sites represent the point of genesis (I), a point roughly halfway along the dune's length (II), and its terminus (III). The longitudinal profile traverses two fossil transverse dunes which are topographically subdued and rarely discernible in the field. The linear dunes have migrated longitudinally over these features. The transverse dunes probably acted as sediment sources over time, and may also have reactivated at the same time as the linear dunes were forming (e.g. Fitzsimmons et al., 2007a). The sedimentology of the three linear dune sites is very similar. Substantial proportions of feldspar and rock fragments, and zircon provenancing studies, suggest that sediments across this region were derived from the basement rocks of the Flinders Ranges to the west (Fitzsimmons et al., 2009; Pell et al., 2000). Feldspar content is especially high at sites that overlie transverse dunes, and within the transverse dunes themselves, however there is no systematic variation in feldspar content with distance from the Lake Frome playa or Flinders Ranges at this scale (Fitzsimmons et al., 2009). This suggests that dune formation may rely on local windrifting. Site I contains clay pellets, which were deflated from adjacent, periodically-inundated, swales. The instability of clay pellets under long distance transport further supports the hypothesis for local windrifting as the dominant mechanism for linear dune formation. Optically stimulated luminescence (OSL) dating was undertaken on samples from each of the stratigraphic units identified at sites II and III, and at regular depth intervals for site I (Fitzsimmons et al., 2007b). The oldest reliable ages for the longitudinal transect occur at site I and indicate activity between approximately 28-34 ka (Figure 2). The three ages are within 2σ of one another and effectively represent deposition of at least 3.2 m of sediment in a single event. The uppermost age at site I lies within 2σ of all the reliable linear dune ages at the other sites (Figure 2), implying effectively coeval aeolian activity at all three sites between approximately 16.5-13.6 ka. The lowermost ages at site I predate those at site II, which may indicate downwind transport over time, but this is unlikely given the age of ~52 ka from the lowermost linear dune horizon at site III further downwind. In the case of this transect in the south-western Strzelecki Desert there is no unequivocal evidence for a downwind decrease in age, as would be expected for dune development by downwind extension. Nor does the sedimentological evidence suggest a decrease in feldspar content downwind, which might also occur with downwind extension. Furthermore, thermoluminescence (TL) dating of linear dunes in the Simpson Desert to the north supports local windrifting. Luminescence dating is therefore a useful tool which assists with the testing of previous empirically unassessed hypotheses for geomorphic processes, since the chronologic evidence is consistent with the sedimentological evidence of locally derived material. Based on the combined evidence of geochronological, sedimentological and stratigraphic methods, the windrift model of local reworking and accretion is favoured.